1. Technical Field
The present invention relates generally to a cable modem; and, more particularly, it relates to a cable modem having a programmable media access controller.
2. Related Art
In recent years, cable television networks have become more widespread. A typical cable TV system can carry many television stations, and is effectively a high bandwidth system. Because of the increasing availability of cable television infrastructure, the use of television cables as the medium for computer data networks has the potential for giving users high bandwidth at a reasonable cost. A cable TV system, however, requires several enhancements in order to function as a data network.
In its classic form, a cable TV system carries information in only one directionxe2x80x94from the cable system headend to the individual user. The user interface to the system generally comprises a receiver such as a television or a stereo. The headend transmits television or stereo channels simultaneously. In general, the user has no influence on what is transmitted and can only choose among the channels the headend is transmitting.
In contrast, a data network carries data from the headend to the user (the downstream path) and from the user to the headend (the upstream path). The individual user requires equipment, such as a cable modem, that can both receive from the headend and transmit to it. A cable data network must be able to handle many individual users simultaneously, each of whom have control over what they receive and transmit.
Cable modems offer greatly improved bandwidth capable of delivering services hundreds, or even thousands, of times faster than conventional modems. Cable modems can achieve data-transfer rates of up to 40 Mbits/s by connecting directly to coaxial lines as opposed to dial-in modems, that use twisted-pair copper telephone lines.
In order for a cable TV network to operate as a data network, it requires a headend capable of both transmitting and receiving data. To ensure that each user receives the data they require, a network protocol must be implemented to allow independent users of the network to utilize the shared headend and the distribution network without interference from or receiving the data of other users.
The network protocol places requirements on both the headend and the user end. Generally, the headend serves as the network controller, and the user""s cable modem must be able to respond to commands from the headend. In cable modems adhering to the well-known OSI reference model, the lowest layer is the Physical layer (PHY), while the next layer up is the Data Link layer. The Data Link layer is segmented into two parts, the Medium Access Controller (MAC), which interfaces with the PHY, and the Logical Link Control (LLC), which interfaces to the MAC and to higher layers. In general, the MAC and LLC provide the following Data Link functionality: transmit and receive data encapsulation, including framing (frame boundary delimitation, frame synchronization), addressing (management of source and destination address), and error detection (detection of physical medium transmission errors); and media access management, including collision avoidance and handling. A physical address or MAC address is a unique Data Link layer address that is assigned to every port or device that connects to a network. Other devices in the network use these addresses to locate specific ports in the network and to create and update routing tables and data structures.
In an effort to coordinate the development of multimedia high-speed data services and the interoperability of network devices, cable operators have formed the Multimedia Cable Network Systems (MCNS) Group in cooperation with the industry research and development consortium CableLabs. The MCNS group has promulgated the Data Over Cable Service Interface Specification (DOCSIS). Other standards, such as DAVIC/DVB have likewise been created. Such standards continue to evolve over time, with the frequent inclusion of additional feature sets.
Previously, integrated cable modem devices have only included physical-layer functions and a fixed-function MAC. These devices are generally compliant with a single specification or a version of a specification. Thus, any changes to the underlying specification require hardware modifications for the MAC to be compliant, resulting in lengthy and expensive product development cycles.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.
Briefly, the present invention relates to a cable modem having a programmable media access controller (MAC). In one embodiment of the invention, a single cable modem device is provided that includes all necessary MAC functions. The cable modem device advantageously allows the MAC functions to be programmed to support evolving standards (e.g., DOCSIS) without requiring expensive hardware upgrades. The cable modem device may also include data-conversion components, a complete PHY that is compliant with both United States and European standards, an Ethernet MAC, a Universal Serial Bus (USB) transceiver, an encryption engine(s), key memory components and other peripheral functions.
The cable modem device of a disclosed embodiment of the invention utilizes a bifurcated microprocessor architecture in which first processing circuitry (e.g., an embedded processor core) is programmed to implement the desired MAC functionality for processing information flowing to and from cable media interface circuits. A second embedded processor core or host system processor provides operating system functionality and controls the boot process for the first embedded processor core. In a further embodiment, separate processor cores are provided for implementing MAC functionality for the downstream and upstream data paths, respectively. The embedded microprocessor cores may be Advanced RISC Machines (ARM) microprocessors or any other suitable microprocessor cores.
In a disclosed embodiment of the invention, cable media interface circuitry, as well as other peripheral circuitry, are coupled to a peripheral bus. The peripheral bus is linked by a bridge circuit to a system bus. The processing circuitry of the programmable MAC is communicatively coupled to the system bus. A novel centralized DMA controller is provided to direct transfer of data between the peripheral bus and the system bus as determined, at least in part, by the programmable MAC.
A cable modem device having a programmable MAC according to the present invention provides a software upgrade path to permit support for new versions of standards as they are adapted. Further, the programmable nature of the cable modem device permits individual manufacturers to differentiate products at the MAC layer without having to modify or replace hardware.
Other aspects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.